QuiltnNan...make your layout 8 x 8 blocks and you will have your central square.
In Borders, make them the width of the blocks (ie 6" border for a 6" finished HST block). Your border style selection is not Long Horizontal, but Blocks from the Drop Down menu. Then list how many blocks you want in the horizontal and vertical borders (at the bottom of the Borders Dialog box). If you need additional borders to increase overall quilt size you can...you can make the vertical borders narrower or 0" and the top and bottom the width you need. I hope this is clear enough for you.